An electric power supply is a device or system that is capable of providing electrical energy to a load-typically by converting electrical energy from one form to another to make the energy compatible with the load's requirements. For example, an electric power supply might convert 120 or 240 volt alternating current (“AC”) energy to lower-voltage, regulated direct current (“DC”) energy appropriate for use by an electronic device such as a computer system. Sometimes power supplies are integrated with the devices for which they supply energy. In other applications, power supplies are discrete components and can be internal or external to the load.
For loads that require a high degree of fault tolerance, it is common to employ redundant power supplies. In prior art redundant power supply systems, plural power supplies drive the load simultaneously. Two general categories of redundant power supply systems exist: power-supply redundant systems and AC-redundant systems. In power-supply redundant systems, the plural power supplies in combination have more than enough capacity to drive the load such that, in the event one of the power supplies fails, the remaining power supplies can continue to drive the load adequately. Thus failure of one power supply in a power-supply redundant system does not interrupt service to the load. A variety of prior art techniques exist for causing the redundant supplies in such a system to share the load's current burden relatively equally. In AC-redundant systems, more than one group of power supplies are employed, wherein each group takes its input from a different source. In an AC-redundant system that employs two such groups of power supplies, twice as many power supplies than needed are present. Use of the term “redundant power supply systems” herein is meant to include both power-supply redundant systems as well as AC-redundant systems.
Switching electric power supplies (also known variously as switch-mode power supplies, switched-mode power supplies and other similar terms) are those that make use of active switching circuitry along with inductive elements to accomplish the energy conversion task with a minimum of energy loss. A common configuration in switching power supplies is to create an internal DC voltage supply that is provided to one or more switching DC-DC output converters for generating desired DC output levels. The DC output levels generated by the output converters are used for driving one or more loads external to the supply. In power supplies that employ this configuration, the internal DC voltage supply that feeds the output converters can be generated from external AC mains or from an external DC supply. For power supplies that take their input from AC mains, rectification circuitry (and in some cases a switching DC-DC power factor correcting converter) is used to create the internal DC supply from the available AC input. For power supplies that take their input from an external DC source, an input DC-DC converter may be used to create the internal DC supply. Whatever circuitry is used to create the internal DC supply for feeding the output converters will hereinafter be referred to as the “input converter.”
Start-up is a stressful time for prior art switching power supplies. The bus that connects the power supply to its load (and sometimes the load itself) typically represents a large capacitance that must initially be charged before the bus reaches the nominal output voltage of the power supply. During the time that this capacitance is being charged, power conversion circuits within the power supply may be operating at their maximum outputs, and current transients out of the power supply can be so large as to trigger short circuit protection mechanisms. To address these problems, prior art power supplies have been equipped with soft start circuitry. Soft start circuitry is intended to bring the output voltage of the power supply up to its nominal output level gradually, so as to avoid the harmful current transients that would result if the output voltage were brought up to the nominal level immediately. Soft start is generally accomplished by gradually increasing an internal reference voltage against which an output voltage (or a scaled version thereof) is compared. The interval of time during which the internal reference voltage is raised from its initial value to its nominal value will be referred to herein as the “soft start time interval.”
Prior art switching power supplies also include the ability to enable or disable an output converter while the other components of the power supply are operating. In prior art power supplies that possess this capability, however, the soft start circuitry is always reset when the output converter is disabled. Thus, when the output converter is later re-enabled, the soft start circuitry will be activated so that the internal reference voltage against which the output converter's output is compared rises slowly over the soft start time interval.